Problem
A flat disk or "wafer" of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough.
The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, material layers applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness. The removal of projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer are referred to as planarization. To this end, chemical mechanical planarization ("CMP") machines have been developed, and are well known in the art (sometimes referred to as chemical mechanical polishing machines), to provide controlled planarization of semiconductor wafers and layers deposited on the wafers.
A typical CMP machine generally includes one or more wafer carriers which retain, carry and control the movement of a wafer to be planarized and which press the front face of the wafer against the surface of a polishing pad. It is important that the wafer carrier is moved with respect to the polishing pad in a manner that allows all areas of the wafer to be polished uniformly.
The polishing pad is often moved at high speeds with respect to the wafer. Polishing pads are typically used on polishing platens that are rotated, as in U.S. Pat. No. 5,498,196, or orbited, as in U.S. Pat. No. 5,554,064, but can also be used in linear belt systems, as in U.S. Pat. No. 5,692,947, or in rotary drum systems as in U.S. Pat. No. 5,707,274 or 5,643,056 or on any other machine that causes relative motion between the wafer and the polishing pad.
Although polishing pads have been developed that do not require the use of an abrasive slurry, a slurry is typically introduced between the wafer and polishing pad to enhance the removal rate of material from the wafer. A common problem is that the wafer will often hydroplane on the slurry causing the wafer to tilt and make uneven contact with the polishing pad due to the relative motion of the wafer and polishing pad. This tilting causes the leading edge of the wafer to be polished at a different rate than the rest of the wafer. Wafers are thus usually rotated during the planarization process to provide more uniform polishing thereof. Although rotation of the wafer tends to average the unevenness of the planarization over the entire wafer edge, the mere rotation of the wafer is not sufficient to produce a wafer which is uniformly planar.
Another problem with existing wafer planarization methods is that the polishing pad wears faster in areas that have longer periods of contact with the wafer. A common problem is for the polishing pad to become concave due to excessive wear in areas having the greatest periods of contact with the wafer. Some of the prior art has attempted to reduce this problem by oscillating the wafer across the polishing pad, moving the wafer in a variable path, as in U.S. Pat. No. 5,549,511 or moving the wafer in a series of arcs, as in U.S. Pat. No. 5,759,918.
However, sudden changes in direction of the wafer carrier cause the carrier and attached wafer to tilt down in the direction of movement across the polishing pad. In addition, the wafer tends to tilt back and forth as it attempts to align itself to the polishing pad after each change indirection is initiated. This tilting occurs in prior art methods even though sophisticated gimbals, membranes or other means are used to keep the wafer parallel to the polishing pad.
An additional problem is presented by wafer planarization systems which move a wafer across a polishing pad in non-linear patterns. In typical prior art systems, a curve to be traced, or tracked, by the wafer carrier is broken down into small line segments which are stored in memory in the motion controller for the wafer carrier. A complex curve may have numerous line segments and each of these segments requires a large number of data points to accurately describe the particular curve in a typical numerical controller using `G-code`, which was an early industry standard. In operation, a large volume of code must be executed in a relatively short time by the numerical controller. Because the time required to execute a block of code is typically shorter than the time required to transfer and process the code, the controller is often unable to timely respond to control commands in high-speed applications.
NURBS (`Non-Uniform Rational B-Splines`) has replaced G-code as an architecture for describing three-dimensional surfaces, and has become the de facto industry standard for the representation and data exchange of geometric information on parts in machine tools. NURBS is based on a generalization of non-rational B-splines and non-rational and rational Bezier curves and surfaces. However, one of the drawbacks of using NURBS is the need for extra data storage (as compared to basic Bezier curves) to define common geometrical shapes (e.g. circles). This extra storage requirement results from NURBS' use of supplementary parameters in addition to the control points. NURBS-shapes are not only defined by control points; weights, associated with each control point, are also necessary. Thus, NURBS, while providing an improvement over G-code, still requires that a significant amount of data be stored and then transferred in real-time during a planarizing operation.
Therefore, a method is needed for planarizing a wafer against a polishing pad that uses an easily generated and compactly stored geometric pattern which evenly wears the pad and evenly planarizes the wafer by minimizing wafer tilting during the polishing process.